For example, a nuclear power plant that includes a pressurized water reactor (PWR) uses light water as a nuclear reactor coolant and a neutron moderator while keeping the light water as high-temperature and high-pressure water which is not boiled throughout a reactor core, sends the high-temperature and high-pressure water to a steam generator so as to generate a steam by a heat exchange operation, and sends the steam to a turbine generator so as to generate electric power.
In such a nuclear power plant, there is a need to periodically inspect various structures of the pressurized water reactor in order to ensure sufficient safety or reliability. Then, when a problem is found after various inspections, a necessary portion involved with the problem is repaired. For example, in the pressurized water reactor, a nuclear reactor vessel body is provided with a plurality of instrumentation nozzles penetrating a lower end plate. Further, each of the instrumentation nozzles is formed so that an in-core instrumentation guide pipe is fixed to the upper end thereof inside the reactor and a conduit tube is connected to the lower end thereof outside the reactor. Then, a neutron flux detector capable of measuring a neutron flux is insertable from the instrumentation nozzle to a reactor core (a fuel assembly) through the in-core instrumentation guide pipe by using the conduit tube.
The instrumentation nozzle is formed in a manner such that an in-core instrumentation cylinder is welded while being fitted into an attachment hole of a nuclear reactor vessel body. For that reason, there is a possibility that a tensile stress may remain in the in-core instrumentation cylinder, the welding portion of the in-core instrumentation cylinder, and the vicinity thereof. Thus, there is an increase in the possibility of stress corrosion cracking due to the long-term use. Here, as the related art, a water jet peening technique is known which prevents stress corrosion cracking by solving a residual tensile stress of a surface by a residual compressive stress. In the water jet peening operation, high-pressure water including cavitation air bubbles is jetted to a surface of a metal member under water so as to solve a residual tensile stress of the surface of the metal member by a residual compressive stress. As such a water jet peening apparatus, for example, an example is disclosed in Patent Literature 1 as below.